This invention relates in general to devices for generating metallic ion beams and, more particularly, to an ion beam generator which is capable of switching among a plurality of cathodes.
A number of different devices have been developed for generating metallic ion beams. One system, as described in U.S. Pat. No. 3,566,185 uses a pair of parallel cathodes spaced from a slit anode. The cathodes are at the same negative potential with respect to the anode. Electrons which are created by the ionization of gas atoms are accelerated toward the anode, but are constrained by the magnetic field and oscillate between the cathodes. Positive ion bombardment sputters material from the cathode to form a plasma from which ions move through the anode slit as a metallic ion beam. This system tends to have relatively low ion current and changing the cathodes to change the metal being ionized is slow and time consuming, requiring release of the vacuum, replacing cathodes and restoring the chamber vacuum.
Brown in U.S. Pat. No. 4,714,860 describes an ion beam generating apparatus. A cathode constructed from the metal to be used as the source of ions is placed in a vacuum chamber spaced from an anode having a single central opening. An electrical potential is imposed between anode and cathode. An electrical arc is generated between anode and cathode, vaporizing a portion of the cathode and forming a metal ion plasma which is moved by a magnetic field toward and through the anode opening toward a target. While this apparatus will produce an effective ion beam, in order to change the metal being transmitted requires substantial disassembly of the apparatus, with release and reformation of the vacuum required. Only a single cathode may be used, aligned with the anode opening. Also, as the cathode erodes with use, efficiency falls off to the point at which the assembly must be disassembled for cathode replacement.
Attempts have been made to incorporate plural cathodes on a rotatable turret so that cathodes of different metals or new cathodes could be rotated into alignment with the anode opening when a cathode is worn or a different metal is desired. However, problems arise with leakage at seals between the rotatable turret and the vacuum chamber, the cathodes not in use may interact with the one in use and alignment with a narrow cathode opening may not be precise.
Thus, there is a continuing need for improved ion beam generating devices capable of rapid and convenient switching between cathodes without impairing vacuum integrity of the system, capable of compensating for erosion of a cathode during use and capable of operating without precise cathode to anode opening orientation and having reduced mechanical and manufacturing complexity.